1. Field of the Invention
The present invention relates to a display device.
2. Description of the Related Art
Active matrix display devices in which transistors are formed over substrates as pixel switching elements have been developed, and display devices in which polycrystalline silicon is used as semiconductor layers of transistors that drive and control display elements have been put into practical use.
However, polycrystalline silicon is formed in such a way that amorphous silicon is formed by a film formation method and a crystallization process is performed. Therefore, variations of a crystallization state affect characteristics of transistors which are formed. Accordingly, a crystallization condition and a crystallization method such that polycrystalline silicon with more uniform crystallized state is obtained have been researched (e.g., refer to Reference 1: Japanese Published Patent Application No. 2002-124467).
In Reference 1, at the time of laser beam irradiation as a crystallization process, an oxide film is formed over an amorphous silicon film, and effects due to a change and a variation of energy in the laser beam are reduced at the time when crystallization of the amorphous silicon film is performed.